1. Field of the Invention
The present invention relates to an optical apparatus, such as a video camera, which can perform a still image shooting operation based on a fixed F-number using a mechanical shutter and an electronic shutter.
2. Description of the Related Art
Advanced video cameras have the capability of shooting high resolution still images in addition to the capability of shooting moving images. The still image shooting operation of the video camera is generally realized by combining a mechanical shutter and an electronic shutter. The electronic shutter can control the start timing and the reset timing of storing an electric charge in an image sensor.
With the combination of a mechanical shutter and an electronic shutter, even a video camera equipped with an image sensor which cannot perform interlace shooting can realize a progressive still image shooting operation (see Japanese Patent Application Laid-Open No. 2003-169248). The light exposure in the still image shooting mode can be expressed by the quantity of light exposure in a duration from the reset off timing of an image sensor to the complete closure of the mechanical shutter.
The electronic shutter, due to its nature, can quickly reset the light quantity within a short period of time. On the other hand, the mechanical shutter requires a relatively long time to completely close. Therefore, the light quantity gradually changes according to the motion of the mechanical shutter. In this case, the shutter time and speed represent time and speed during the period of time from the reset off of the image sensor to the complete closure of the mechanical shutter.
An actual exposure time can be defined as a total light exposure in the duration from the shutter opening action start timing (i.e., the reset off timing of the image sensor) to the complete closure timing of the mechanical shutter, normalized by a unit light exposure in an opened state.
The actual exposure time is generally dependent on the action pattern and individual differences of the mechanical shutter, and accordingly, the light exposure varies depending on each shutter. Therefore, accurately obtaining an actual exposure time is important. As discussed in Japanese Patent No. 3041543, the actual exposure time can be calculated in the following manner.
The optical apparatus disclosed in Japanese Patent No. 3041543 does not include an electronic shutter. Instead, a mechanical diaphragm performs an opening/closing action to realize a shutter function. In this case, the actual exposure time can be obtained based on an aperture area of the diaphragm and the time required for the shutter opening/closing action.
If the shutter opening/closing action is a uniform accelerated motion, the light exposure during the shutter action changes along a trapezoidal area. However, if the opening action and the closing action are mutually different, an actual change of the light exposure may deviate from a trapezoidal area.
Furthermore, in a system including a combination of a mechanical shutter and an electric shutter, an opening action is performed by the electronic shutter and a closing action is performed by the mechanical shutter causing a uniform accelerated motion. Accordingly, the transitional change of the light exposure cannot be expressed by a trapezoidal area. Thus, using the approximation with a trapezoidal area will cause errors.
Moreover, a stepping motor (hereinafter referred to as an “STM”) can be used to drive a shutter. The STM is generally configured to allow counting the number of control pulses to measure a shifting amount of the STM. Thus, the STM can control an aperture area of the shutter.
Accordingly, when the momentary light exposure changes along a predetermined trapezoidal area, the actual light exposure can be obtained based on the aperture area and the time required for the shutter opening/closing action. However, a simple mechanical shutter, if equipped with no STM, cannot detect an intermediate position of the shutter during a shifting operation. Thus, the simple mechanical shutter can only rely on mechanical contact to detect opened and closed positions.
Furthermore, unequivocally controlling the action of a mechanical shutter is difficult. Thus, the conventional method for calculating the actual exposure time cannot be directly applied to this kind of mechanical shutter. Furthermore, to improve the shutter speed, the type of a mechanical shutter may be changed from a uniform motion type to an accelerated motion type. However, the conventional calculation method cannot be applied to the accelerated motion type. As described above, when a mechanical shutter having a simple control mechanism and causing accelerated motion is used, accurately calculating the actual light exposure is difficult.